To remove harmful substance such as particulate matter (PM) and NOx (nitrogen oxide), HC (hydrocarbon), CO (carbon monoxide), SOx (sulfur oxide), etc. from exhaust gases discharged from internal engines of construction machines, industrial machines, etc., ceramic honeycomb structures are used as carriers for catalysts for cleaning exhaust gases, and filters for capturing fine particles. As shown in FIGS. 1(a) and 1(b), a ceramic honeycomb structure 1 comprises large numbers of cells 13 partitioned by porous cell walls 12 and extending in an exhaust-gas-flowing direction. A ceramic honeycomb structure used as a catalyst carrier carries an exhaust-gas-cleaning catalyst on its cell wall surfaces and in pores inside the cell walls, so that an exhaust gas passing through the ceramic honeycomb structure is cleaned by the catalyst.
When an exhaust gas is cleaned by such a ceramic honeycomb structure, harmful substance in the exhaust gas should come into efficient contact with a catalyst carried by cell walls to improve cleaning efficiency. Generally conducted to this end is to increase a cell density by decreasing the opening areas of cells, thereby obtaining a large contact area with a catalyst. However, increase in the cell density results in increase in pressure loss.
To solve such problem, JP 2006-517863 A discloses a catalyst support comprising a porous ceramic honeycomb structure having pluralities of parallel cells penetrating from the inlet ends to the outlet ends, which has porosity of more than 45% by volume, as well as a network structure having communicating pores having a narrow pore diameter distribution having a median pore diameter of more than 5 μm and less than 30 μm. JP 2006-517863 A describes that this catalyst support (ceramic honeycomb structure) can bear a higher percentage of a catalyst without suffering pressure decrease.
WO 2007/026803 A1 discloses a ceramic honeycomb structure comprising pluralities of cells penetrating between two end surfaces, which are constituted by porous cell walls having large numbers of pores, and plugs for sealing the cells at either end surfaces or inside the cells, the cell walls having permeability of 7×10−12 m2 to 4×10−8 m2. WO 2007/026803 A1 describes that this ceramic honeycomb structure exhibits excellent cleaning efficiency with small pressure loss.
In the ceramic honeycomb structures described in JP 2006-517863 A and WO 2007/026803 A1, however, pores in the cell walls may be clogged by a catalyst applied to the cell wall surfaces and pores in the cell walls, resulting in large pressure loss in a catalyst-carrying ceramic honeycomb structure. As a result, a catalyst carried in pores in the cell walls is unlikely used effectively, failing to obtain improved cleaning efficiency. Particularly in a ceramic honeycomb structure (catalyst carrier) having no plugs, like the catalyst support described in JP 2006-517863 A, an exhaust gas may not be easily flowable through pores in the cell walls, resulting in poor cleaning efficiency. Further, the ceramic honeycomb structure of WO 2007/026803 A1 has poor strength, because its cell walls per se have small strength due to large permeability.